Selective catalytic reduction (SCR) of oxides of nitrogen (NOx) by nitrogen compounds, such as ammonia or urea, can be incorporated into mobile diesel internal combustion engines such as heavy-duty truck and bus engines. Generally, in the ammonia-SCR process, an aqueous water solution of urea is injected into the exhaust gas stream of an engine. At temperatures above 160° C., the urea begins to undergo hydrolysis and thermal decomposition resulting in the production of ammonia. The resulting mixture including urea/ammonia and exhaust gases then passes to an SCR catalyst such as platinum (Pt), vanadium, or zeolite, where the ammonia reacts with NOx gases to form nitrogen gas and water.